The present invention relates to improvements in lift truck mounted clamping apparatus for picking up, transporting and stacking large rolls of paper such as newsprint and kraft paper. More particularly the invention relates to improvements in the hydraulic piston and cylinder assemblies utilized to open and close paper roll-handling clamps of the pivoted arm type, as exemplified by Sinclair U.S. Pat. No. 3,896,957 and Esser U.S. Pat. No. 3,905,635.
In the past, it has been recognized that it is necessary for a lift truck mounted paper roll clamp to have the capability of an exceptionally wide range of movement of the clamp arms relative to one another, so that they may be used to engage rolls of paper of widely varying diameter. Ideally, it is in fact desirable for the clamp arms to be able to close completely with respect to one another to pick up scrap paper, as well as to separate sufficiently to carry rolls of paper of the largest diameter, in the range of about sixty inches.
Particularly with clamp arms of the type wherein at least one clamp arm pivots with respect to the other clamp arm to open or close the clamp selectively, achievement of this wide range of movement by means of conventional double-acting piston and cylinder assemblies is difficult because competing design factors peculiar to lift truck paper roll clamps limit the extent to which conventional techniques of maximizing motion from piston and cylinder assemblies can be employed. For example the conventional technique of reducing the length of the lever arm through which the piston and cylinder assembly operates a pivoted clamp arm, to give a greater range of movement of the clamp arm's extremity relative to the linear extension and contraction of the cylinder assembly, results in a lesser clamping force exerted by the clamp arm in the absence of other system changes. Thus, although a relatively short piston and cylinder assembly could be connected by a short lever arm to give a wide range of clamp arm movement, the clamping force available for gripping large, heavy rolls of paper would be reduced.
The hydraulic pressure could theoretically be increased to compensate for the foregoing geometric loss of clamping force, but such a solution is impractical in the field of lift truck load handling attachments which do not have their own integral hydraulic fluid sources but rather must be interchangeably adapted for use with the existing hydraulic systems of lift trucks.
Alternatively, the piston and cylinder assembly diameter might be increased to recover the clamping force lost by the reduction of lever arm length. A larger diameter piston and cylinder assembly however requires a correspondingly thicker clamp arm if the cylinder is to be enclosed at least partially within the clamp arm, as is desirable with clamp arm cylinders of the type which do not extend transversely of the lift truck. A thicker clamp arm is highly disadvantageous when moving rolls of paper which are stacked with their axes vertical, since clearance between these vertically stacked rolls is often small and a thick clamp arm forcibly inserted between adjacent rolls therefore presents a serious risk of scuffing or tearing of the surface layers of paper.
Additional range of clamp arm motion could alternatively be achieved, without presenting the difficulties outlined above, by the use of a longer piston and cylinder assembly, but this produces different space problems. With clamp arm cylinders of the type which do not extend transversely of the lift truck but rather slant in a forward direction, a longer piston and cylinder assembly must be attached to a point located rearward of the pivot point of the clamp arm, as shown for example in Sinclair U.S. Pat. No. 3,896,957. This places the piston and cylinder assembly in the area of the rotating connection between the clamp arms and the elevatable load supporting carriage of the lift truck, which complicates the rotator structure and requires a relatively large forward dimension of the rotator, the latter tending to mount the clamp arms in an excessively forward position decreasing the weight-carrying capacity of a counterbalanced lift truck.
The length of a pivoted clamp arm piston and cylinder assembly of the type which extends transversely of the lift truck, as exemplified by the Esser U.S. Pat. No. 3,905,635, may be maximized in order to maximize the range of clamp arm motion, but the transverse placement of the piston and cylinder assembly inherently tends to require the carrying of the load in an excessively forward position which, as in the previous paragraph, decreases the weight-carrying capacity of a lift truck.
What is required therefore is an improved piston and cylinder structure for maximizing the range of movement of pivoted-type clamp arms without sacrificing needed clamping force or lift truck capacity, without requiring higher than normal hydraulic pressure, and without interfering with the slenderness of the clamp arms or with the space normally occupied by the clamp rotator structure.